The invention relates to an electronic clutch control system for automatic control of initial engagement of a friction clutch between an engine and transmission of a motor vehicle during take-off of the vehicle from rest.
According to our ealier UK published patent Application No. 2 088 007 A an electronic control system for a friction clutch in an engine driven vehicle for starting the vehicle from rest comprises an engine speed sensor for deriving an engine speed signal, a reference signal generator for generating a reference signal equivalent to an engine speed signal at a reference engine speed, a clutch actuation system responsive to the engine speed signal to move the clutch towards an engaged state when the engine speed exceeds the reference speed and thereby load the engine to hold down the engine speed to the reference speed.
It has already been proposed in UK published Patent Application No. 2 079 889 A to integrate an error signal derived from the engine speed signal and reference signal and to use this integral to control the engagement of the clutch. The integrated error signal is unsuitable for bringing the clutch from a fully disengaged position to a position where it is about to bite because the sense of the integral signal continues to command full clutch disengagement until after the engine speed has reached the reference speed and has exceeded the reference speed by such an amount and for such a time as to change the sign of the integral. Thereafter, further time is taken until the clutch has moved to the bite position. During this time, substantial engine overspeed occurs and initial clutch engagement tends to be harsh. To overcome this problem, it is also proposed in the said UK patent application No. 2 079 889 A to utilise the error signal itself to control clutch engagement until the engine speed reaches the reference speed and thereafter to use the integral to control clutch engagement.
If the transition from using the basic signal to using the integral does not occur just as the clutch begins to bite, there is still a tendency for transient effects to result in harsh initial clutch operation. If the point of changing over to use of the integral is set accurately for a new clutch, subsequent wear of the clutch alters the value of the clutch position signal at the clutch bite position and again results in the transient effects causing harsh clutch engagement.
It has also been proposed in European published patent application No. 0 059 035 to derive a clutch wear compensation signal from the response of the control system to clutch take-up and to add the wear compensation signal to the clutch position signal to provide a modified clutch position signal compensated for wear.
Whilst this proposal was found to improve the smoothness of operation of the clutch, some harshness in clutch operation could still occur.
We have now discovered that it is desirable to derive and utilise a clutch condition compensation signal in such a way as to compensate for changes which occur in the torque-capacity/clutch-position characteristic of the clutch.